IO in action

The IO type serves as a tag for operations (actions) that interact with the outside world. The IO type is abstract: no constructors are visible to the user. [...]

The Haskell 2010 Report (page 95 of 329).

Instead of that conventional approach:

data IO  -- abstract

getChar ::         IO Char
putChar :: Char -> IO ()
         ⋮

describe IO using other types, ones with no visible constructors:

data (->) a b  -- abstract
data OI        -- also abstract

type IO a =         OI -> a
getChar ::          OI -> Char  -- an I/O action
putChar :: Char -> (OI -> ())   -- a function with one parameter, whose result is an I/O action
         ⋮

Starting up

Main.main is also an I/O action:

main :: OI -> ()

Therefore an internal subroutine in the Haskell implementation provides each running program with an initial OI value. However, most programs will require more than just one:

getLine         :: OI -> [Char]
getLine u        = let !(u1, u2) = partOI u in
                   let !c = getChar u1 in
                   if c == '\n' then
                     []
                   else
                     let !cs = getLine u2
                     in c:cs

putLine         :: [Char] -> OI -> ()
putLine (c:cs) u = let !(u1, u2) = partOI u in
                   let !_ = putChar c u1 in
                   putLine cs u2
putLine []     u = putChar '\n' u

So another I/O action is needed in order to access that same internal subroutine from Haskell:

partOI :: OI -> (OI, OI)

If more than two new OI values are needed:

partsOI :: OI -> [OI]
partsOI u = let !(u1, u2) = partOI u in u1 : partsOI u2

But why are these abstract OI values needed at all - what purpose do they serve?

Actions and functions

Looking more closely at getLine, putLine and partsOI reveals an interesting fact:

• each OI value is only used once (if at all).

Why is this important? Because in Haskell, functions have their basis in mathematics. That imposes certain requirements on function, including this one:

• if a function's result changes, it is only because one or more of it's arguments has changed.

If they're always used with different OI values, then I/O actions can be used like functions:

partsOI :: OI -> [OI]
partsOI = unfoldr (Just . partOI)

even if they're defined using subroutines:

foreign import "oi_part"    partOI  ::         OI -> (OI, OI)
foreign import "oi_getchar" getChar ::         OI -> Char
foreign import "oi_putchar" putChar :: Char -> OI -> ()

The need for an OI value also helps to prevent I/O actions from being used as subroutines:

trace :: [Char] -> a -> a
trace msg x = case putLine msg of !_ -> x  -- how is this supposed to work?

Monadic actions

The monadic interface:

instance Monad ((->) OI) 
    return x =  \ u -> let !_ = partOI u in x 
    m >>= k  =  \ u -> let !(u1, u2) = partOI u in
                       let !x = m u1 in
                       let !y = k x u2 in
                       y

allows getLine and getLine to be defined more compactly:

getLine :: OI -> [Char]
getLine = do c <- getChar
             if c == '\n' then
               return []
             else
               do cs <- getLine
                  return (c:cs)

putLine :: [Char] -> OI -> ()
putLine []     = putChar '\n'
putLine (c:cs) = putChar c >> putLine cs

and conceals the use of all those OI values. But not all definitions will benefit from being monadic:

partsOI :: OI -> [OI]
partsOI = do (u1, u2) <- partOI; return (u1 : partsOI u2)

Further reading

• Merely monadic provides more information about Haskell's implementation of the monadic interface.

• For those who prefer it, John Launchbury and Simon Peyton Jones's State in Haskell explains the state-passing approach currently in widespread use.